flight_mode_manager: Extracted position trajectory motion planning into a library

Extract the functionality to plan smooth position motion trajectories into a
motion planning library, such that it can be used in other parts of the code as well.

.clang-tidy

@@ -82,6 +82,7 @@ Checks:          '*,
                   -modernize-use-equals-delete,
                   -modernize-use-override,
                   -modernize-use-using,
+                  -modernize-use-trailing-return-type,
                   -performance-inefficient-string-concatenation,
                   -readability-avoid-const-params-in-decls,
                   -readability-container-size-empty,